A. Wireless Location
Early work relating to Wireless Location Systems is described in U.S. Pat. No. 5,327,144, Jul. 5, 1994, “Cellular Telephone Location System,” which discloses a system for locating cellular telephones using time difference of arrival (TDOA) techniques. This and other exemplary patents (discussed below) are assigned to TruePosition, Inc., the assignee of the present invention. The '144 patent describes what may be referred to as an uplink-time-difference-of-arrival (U-TDOA) cellular telephone location system. The described system may be configured to monitor control channel transmissions from one or more cellular telephones and to use central or station-based processing to compute the geographic location(s) of the phone(s). TruePosition and others have continued to develop significant enhancements to the original inventive concepts. An example of a U-TDOA WLS is depicted in FIG. 1. As shown, the system includes four major subsystems: the Signal Collection Systems (SCS's) 10, the TDOA Location Processors (TLP's) 12, the Application Processors (AP's) 14, and the Network Operations Console (NOC) 16. Each SCS is responsible for receiving the RF signals transmitted by the wireless transmitters on both control channels and voice channels. In general, an SCS (now sometimes called an LMU, or Location Measuring Unit) is preferably installed at a wireless carrier's cell site, and therefore operates in parallel to a base station. Each TLP 12 is responsible for managing a network of SCS's 10 and for providing a centralized pool of digital signal processing (DSP) resources that can be used in the location calculations. The SCS's 10 and the TLP's 12 operate together to determine the location of the wireless transmitters. Both the SCS's 10 and TLP's 12 contain a significant amount of DSP resources, and the software in these systems can operate dynamically to determine where to perform a particular processing function based upon tradeoffs in processing time, communications time, queuing time, and cost. In addition, the WLS may include a plurality of SCS regions each of which comprises multiple SCS's 10. For example, “SCS Region 1” includes SCS's 10A and 10B that are located at respective cell sites and share antennas with the base stations at those cell sites. Drop and insert units 11A and 11B are used to interface fractional T1/E1 lines to full T1/E1 lines, which in turn are coupled to a digital access and control system (DACS) 13A. The DACS 13A and another DACS 13B are used for communications between the SCS's 10A, 10B, etc., and multiple TLP's 12A, 12B, etc. As shown, the TLP's are typically collocated and interconnected via an Ethernet network (backbone) and a second, redundant Ethernet network. Also coupled to the Ethernet networks are multiple AP's 14A and 14B, multiple NOC's 16A and 16B, and a terminal server 15. Routers 19A and 19B are used to couple one WLS to one or more other Wireless Location System(s).
FIG. 1A depicts the components representative of a standard wireless communications system (WCS) 100, which may take the form of a cellular telephone network or the like. Although the technology represented in FIG. 1A is expressed with some of the terminology typical of a Global System for Mobile Communications (GSM) infrastructure, the technology is also comparably applicable to and beneficial for implementations of cellular wireless communications in accord with other standards, such as the Third Generation Partnership Project (3GPP) technical specifications describing the Universal Mobile Telecommunications Service (UMTS). In FIG. 1A, the wireless mobile communications unit or mobile station (MS) 101 communicates via a radio frequency (RF) link carrying transmissions to and from a base transceiver station (BTS) 102. As highlighted in the dashed circle in FIG. 1A, the BTS facilities include the uplink-receive (U_Rx) and downlink-transmit (D_Tx) antenna(s) and associated cables for the appropriate signals carrying the wireless communications. A set of (typically three) BTS cell sectors (or sectorized cellular areas of operation) cover a localized communications area or cell (surrounding a serving BTS) served by the antenna(s) deployed at the BTS terminal location. Each cell sector is identified by its unique cell global identifier (CGI, which term is also used herein to refer to the BTS cell facilities). Each BTS may individually or independently generate its time base or time-standard/reference for its transmitted downlink signals based upon an independent oscillator that operates at a nominal time base frequency, within specification tolerances. For GSM service, a compliant standard BTS timebase reference is specified to operate at 13 MHz, within a tolerance of 0.05 ppm or 0.65 Hz. A set of the various BTSs covering a broader operational region are controlled by a base station controller (BSC) 103. The BSC manages the MSs and BTSs operating within its domain, and this management includes the handover of the responsibility for the integrity of the RF link with a particular MS from one BTS to another, as the MS moves from the cellular coverage of the cells of one BTS to those of the other BTS. In a similar manner at a lower level of communications management, the BSC also manages the handover of an MS from one BTS sector to another and the BTS detects the successful execution of the handovers within its domain. At a higher level of management, a mobile switching center (MSC) 104 manages a multiplicity of BSCs. In supporting the WCS operations, any MS operating under the control of its particular serving CGI (SCGI) is used to synchronize itself to the SCGI's transmitted BTS downlink “beacon” signal, and thus the signals from the distinct BTSs are not required to be synchronized to a common time standard, such as the GPS time base.
FIG. 1B shows a WLS that cooperates as an adjunct to a wireless communications system. In this example, the WLS is called a Serving Mobile Location Center (SMLC) 110. An infrastructure-based, or “overlay,” WLS can be represented with the overlay configuration of components depicted in FIG. 1B. In FIG. 1B, the RF uplink signals in the communications channel from the MS/UE 101 of interest are received and measured by LMUs 112 that are deployed at locations distributed throughout the operational domain of the communications system. (Note regarding terminology: In 3GPP GSM terminology, the term “SMLC” refers to the entire WLS whereas in other contexts “SMLC” refers to the sub-system component that is called a “WLP”. As also used herein, the 3GPP term “LMU” refers to the geographically dispersed SMLC/WLS component that receives transmitted RF signals and measures (e.g., location-related) signal characteristics, whereas such a component may be called the signal collection system “SCS” in other contexts or descriptions of the background art.) Typically, as may be visualized with the “overlay” of FIG. 1B on top of FIG. 1A, LMUs 112 are deployed at BTS 102 facilities, and thus the LMU usually accesses or “taps” its uplink-receive (U_Rx) signals for the location-related measurements via multi-coupling to the same signal feeds that the BTS uses from the antenna(s) deployed for the communications. For time base synchronization of the (location-related) data collections and measurements at the distributed LMU sites, the LMU accesses GPS signals via a GPS-receive (GPS_Rx) antenna with cable, as highlighted in the dashed circle in FIG. 1B. Additionally, the LMU senses the BTS downlink transmissions via a downlink-receive (D_Rx) antenna with cable. As depicted in FIG. 1B, although the LMUs are typically but not necessarily deployed at BTS sites, they are also not necessarily deployed one-for-one with the BTSs. The measurements of the received signal characteristics extracted by multiple LMUs are managed and collected through wireless location processors (WLPs) 203, each of which directs the operations of multiple LMUs. The WLP oversees the selection of the particular LMUs that are tasked with providing the measurements for a particular MS of interest. Upon reception of the appropriately measured signal data, perhaps including through other WLPs managing LMUs not under its direct control, the WLP will typically also evaluate the data and determine the optimal (location) estimate based upon the data. Typically, a WLP may manage the operations of LMUs covering a geographic region for which the corresponding communications services are provided by multiple BSCs. The wireless location gateway (WLG) 114 of the SMLC conducts overall control and tasking of the WLPs. The WLG is typically (but not necessarily) co-located with a MSC 104 (and may interface with it). The WLG interfaces with and exchanges location-related requests, information, or data with the multiple BSCs it serves within the communications system. The WLG validates the location-service requests, and disperses the location-determination results to authorized recipients.
The performance of a U-TDOA WLS (and other location systems) is normally expressed as one or more circular error probabilities. The United States Federal Communications Commission (FCC), as part of the Enhanced 9-1-1 Phase II mandate, requires that network-based systems, such as a U-TDOA system, be deployed to yield a precision that generates a one-hundred meter (100 m or 328.1 feet) accuracy for 67% of emergency services callers and a three-hundred meter (300 m or 984.25 feet) accuracy for 95% of emergency services callers. The requirements for precision vary with the location service deployed, but if the precision (such as predicted by the Cramer-Rao bound for instance) of the U-TDOA location system is such that the location quality of service is exceeded by a deploying fewer LMUs than BTSs, such a deployment would be advantageous because it would reduce the cost of the system.
The inventive techniques and concepts described herein apply to time and frequency division multiplexed (TDMA/FDMA) radio communications systems including the widely used IS-136 (TDMA), GSM, and OFDM wireless systems, as well as code-division radio communications systems such as CDMA (IS-95, IS-2000) and Universal Mobile Telecommunications System (UTMS), the latter of which is also known as W-CDMA. The Global System for Mobile Communications (GSM) model discussed above is an exemplary but not exclusive environment in which the present invention may be used.
B. Problems with Building a Sparse WLS
In a non-sparsed U-TDOA system (a U-TDOA system with 1 LMU per BTS), LMUs are able to detect and demodulate downlink signals (beacons or Broadcast Control Channels (BCCH)) from the resident cell. The measured timing is then compared to system time, determined by the LMU's GPS-based clock, and then sent to the SMLC for storage or forwarding to other LMUs. Each LMU will then be able quickly to demodulate uplink messaging since the channel and timeslot are provided in the location request and the frame timing offset from system time for each adjacent cell and sector is known.
In a sparsed U-TDOA system (a U-TDOA system with a less than 1 LMU per BTS deployment ratio), the increased distances between radio emitter (the mobile device) and the radio receiver (the LMU) resulting from the selective deployment (“sparsing”) will have an adverse effect on U-TDOA location accuracy and will inhibit the LMU's ability to determine frame timing offsets, which are needed in a GSM environment. An LMU, to generate the timestamps needed for TDOA, should: (1) detect and demodulate cell downlink beacons to determine cell timing, and (2) detect and demodulate uplink signals. The requirements that the LMU receive and demodulate both uplink and downlink signals in the presence of noise, adjacent channel interference, co-channel interference and at the distance of several cell radii make it difficult to minimize LMU deployment cost.